For example, when data is recorded onto an optical disc, such as a CD, a DVD, ad a Blu-ray Disc, a tracking process (or tracking control) of a semiconductor laser is performed. As one specific example of the tracking process, a three-beam tracking method disclosed in a non-patent document 1 is listed. The three-beam tracking method is a technology explained as follows. Firstly, a light beam emitted from an optical pickup enters into a diffraction grating, thereby a main beam, which is mainly used for recording data, and two sub beams, which are mainly used for the tracking process, are generated. And then, the sub beams are used to perform the tracking process. The spots of each of the two sub beams are shifted by half of a track pitch, compared with the spot of the main beam, on the recording surface of a single layer type optical disc. The tracking process is performed by detecting a difference in the amount of light (or phase) which is obtained by receiving the reflected light of the two sub beams with a two-divided photo detector or the like.
Non-Patent document 1: “optical head for magneto-optical disk”, Electronic Materials, Kogyo Chosakai Publishing, published on Jul. 1, 1988, vol. 27, no. 7, p 73-74